Background and Objective: Radiation usually results in paranasal sinusitis in patients with nasopharyngeal carcinoma (NPC), which influences patients quality of life. This study aimed to determine the relationships between dose distribution in the nasal cavity and nasal mucous injury in patients with NPC treated by intensity-modulated radiation therapy (IMRT), and to find the tolerable radiation dose for the nasal mucous. Methods: Sixty-six patients with NPC treated by IMRT between October 2006 and November 2008 were enrolled. The irradiation dose in the nasal cavity was determined by the computer with the IMRT work platform. Mucociliary transport rate (MTR) was detected by modified saccharine test before IMRT, at the end of IMRT, and at 3, 6, and 12 months after IMRT. Results: The data were available for 129 nasal cavities. The cavities receiving a mean dose below or equal to 37 Gy showed substantial preservation of nasal mucous after IMRT. The MRT decreased to (62.82 + 38.59)%, (56.78 ± 37.79)%, (64.05 ± 39.37)%,and (71.13 + 39.55)% of pre-IMRT value at 4 time points after IMRT, with significant differences among the data (P ＜0.05). In contrast, when the cavities received a mean dose higher than 37 Gy, no significant differences in MTR among the time points were observed. At 3 months after IMRT, the MTR was the lowest (38.27% of pre-RT value). Conclusions: A mean radiation dose of ≤ 37 Gy for the nasal cavity is an optimal dose to protect the nasal cavity function.